Composite blades developed for commercial aircraft engine fan blades may be constructed of laminated carbon/epoxy "prepreg" material. A "prepreg" is a layer of carbon fibers covered with resin and arranged to form a cloth. Prepreg cloth layers may be layered and cured to form a composite structure. The laminates may experience interlaminar separation under certain circumstances. When laminated fan blades are subject to high energy impacts (e.g., birds, or other foreign objects), the interlaminar separation can result in delamination and a reduction in the blade's structural integrity.
The shear stresses which may tend to delaminate the blade structure are generated when the composite blade is subjected to high twisting and bending loads. These loads normally result from impacts which occur on the leading edge of the blade. When the blade is subjected to an impact, the peak shear stresses tend to be transmitted to the middle of the blade, as well as the leading and trailing edges.
Previous attempts to improve resistance to delamination of composite fan blades have involved, for example, stitching a full size all prepreg blade before cure. Alternatively the blade may have been manufactured of a braided or combination braided/stitched prepreg. It is also known to use a foam or other insert to strengthen a composite blade.